Chris,
1) when display period is change, display unit is determined based on these settings:
timeAxis: {
unitSizePolicy: "max", // or "min". default is "max"
minUnitWidth: 1,
maxUnitWidth: 100
},
what happens is that the chart would try to find the largest display unit that would fit within the new selection. e.g. if you have 1000 pixels and you selected a period of 1 year, chart would look at the area.displayUnits and data.units and start from the largest to see if it can be used to show the chart. so, if we start with 1 year, it would be mean that there would be 1 unit, 1000px in width - as the maxUnitWidth is 100, this year cannot be used as a unit. so, it would traverse the list of the units until it finds the first that match. in this situation it would be a week.
if the unitSizePolicy was "min", it would continue. so, for days - 365 days would imply about 3px per unit which still fits the configuration. moving on to hours would imply 365x24 = 8760 units, which would imply required unit with of 0.11px which is less than the minUnitWidth. so the chart would show the year in days.
Now, when this is clear, you can start to adjust the min/max unit width and policy to achieve the desired effect.
2) Alternative would be a manual control. You can put up your own toolbar items for period selection and then you can build your own logic. So for instance, if you want the year to be in 5 day units rather than in weeks or days, you can achieve it using this approach:
timechart.setDisplayPeriod("max");
f = function(){ timechart.displayUnit("5 d"); timechart.off("animationDone", f)};
timechart.on("animationDone", f);
to explain: setDisplayPeriod changes the displayPeriod to the needed (e.g. 1 year, last year whatever you need) and then disaplyUnit() is called once the animation is completed.
Here is an example:
http://jsfiddle.net/s8ndpwk2/7/
3) You can use area.displayUnits to leave only one unit if you do not need the unit change during the exploration, but make sure that the min and max widths are set properly to accommodate the display requirements.
Janis